1) Field of the Invention
The present invention relates to a technology for sealing a radioactive-material container.
2) Description of the Related Art
When a nuclear fuel assembly is at the end of a nuclear fuel cycle, finishes the combustion, and is not useful any longer, the nuclear fuel is called a recycle fuel assembly. The recycle fuel assembly is cooled at a cooling pit of a nuclear power plant for about 10 years because the recycle fuel assembly contains highly radioactive materials such as fission product (FP) and requires thermal cooling. Then, the recycle fuel assembly is contained in a radioactive-material container, conveyed to a reprocessing facility, and stored. Because the radioactive-material container contains highly radioactive materials, the radioactive-material container must be sealed with a strict care while the radioactive-material container is stored for about 40 years to 60 years.
FIG. 20 is a cross section for illustrating a sealing structure of a conventional radioactive-material container. In a conventional radioactive-material container 600, a primary lid 607 and a secondary lid 608 are fixed using bolts 610, 611 on a flange member 606. From a viewpoint of maintaining a sealing function over a long period of time, metal gaskets 618, 619, which have a heat resistance, a corrosion resistance, and a high durability, are used to seal a space between the primary lid 607 and the flange member 606 and a space between the secondary lid 608 and the flange member 606. That is, using the metal gaskets 618, 619, the recycle fuel stored in the radioactive-material container 600 is sealed.
FIGS. 21A, 21B are enlarged views for illustrating the metal gaskets 618, 619 and a sealing part of the radioactive-material container 600. A sealing part between the primary lid 607 and the flange member 606 and a sealing part between the secondary lid 608 and the flange member 606 has the same sealing structure. A metal groove 625 is formed by machining and the metal gaskets 618, 619 with a double-ring structure are used. In the metal gasket 618 or the metal gasket 619, inner covers 622a, 622b respectively cover coil springs 621a, 621b so as to form two rings, and an outer cover 623 covers the rings. The coil springs 621 and the inner covers 622 are made of Inconel (a registered trademark, a nickel alloy containing 16% chromium and 7% iron), which is corrosion resistant and oxidation resistant at high temperature, and the outer cover 623 is made of aluminum (the reference symbol with a subscript is represented by the reference symbol without the subscript, and the same is applied hereinafter). The metal gasket 618 (619), shown in FIG. 21A, has not been used yet. The metal gasket 618 (619), shown in FIG. 21B, is tightened and transformed by fixing the secondary lid 608, a body 601, and the like, and exerts a sealing function.
The metal gasket 618 (619) is fixed to the gasket groove 625 using a bolt hole arranged in the outer cover 623. As the metal gasket 618 (619), “TRYBACK” from NIPPON VALQUA INDUSTRIES, LTD. or “Helicoflex” from Cefilac in France, which are often used in radioactive-material container for nuclear power, may be used.
A time-and-temperature dependence of both a plastic-deformation ratio and a sealing performance of a metal gasket can be represented by Larson-Miller Parameter (LMP), and the details are disclosed in the documents: KATO, ITO, AND MIEDA, Development of method of verifying the long-term sealing performance of spent fuel storage casks, Journal of the Atomic Energy Society of Japan, 1996, Vol. 38, No. 6, pages 95 to 101. Generally, a long-term sealing performance of a metal gasket is verified by acquiring LMP of sealing-maintenance limit and estimating the limit time at a predetermined temperature.
The radioactive-material container 600, which contains the recycle fuel assembly, is stored in the storage facility for a long time of several decades. At this time, the recycle fuel gives out a decay heat, therefore, the metal gaskets 618, 619 is used in the environment at about 120 degrees at the beginning. Then, the temperature gradually goes down during the storage period of several decades, and at the end of the storage period, the metal gaskets 618, 619 is used in the environment at about 60 degrees. The outer covers 623 of the metal gaskets 618, 619 are made of aluminum, and for aluminum, the temperature range described above is corresponding to a range between a temperature where a high-temperature creep occurs and a temperature where a low-temperature creep occurs. Therefore, even though the metal gaskets 618, 619 have the sufficient sealing performance in the early stage, the stress relaxation is caused by a creep deformation and the sealing performance may go down after the metal gaskets 618, 619 are used in a high-temperature environment for a long time. Moreover, though the metal gaskets 618, 619 can maintain the sealing under the applied stress of a few megapascals (MPa), the outer cover 623 creeps easily when the high stress is applied and the outer cover 623 is stressed strongly. Currently, to maintain a desired sealing performance for several decades, a high material (such as gold and silver), which does not creep easily, is used to the metal gasket, or a metal gasket with a large diameter, which enables maintaining a sealing function even when a creep occurs, is used.
A process of containing the recycle fuel assembly in the radioactive-material container 600 needs to be conducted in a pool. After setting the metal gasket 618 in the radioactive-material container 600 sunk in the pool and lifting up the radioactive-material container 600 from the pool, it is necessary to remove water around the metal gasket 618 by vacuum drying. However, an interspace in the sealing part is small, and in some cases, it takes a long time to completely remove the water inside the metal gasket 618. Japanese Utility Model Laid-Open Publication No. H5-75154 p. 1 FIGS. 1 and 2 discloses a structure that ensures a sealing performance of a metal gasket by forming a cover of solid lubricant between an outer cover and an inner cover. However, such a structure has a problem that, in case the water enters inside the metal gasket accidentally during the process conducted in the pool, it takes a very long time to remove the water completely.